This invention relates to the SZ twisting of elements such as wires or the like in general and more particularly to an improved method and apparatus for avoiding length differences when twisting a large number of elements.
The development in the field of SZ twisting technology for electrical cables and wires, which started at the beginning of the Sixties, has led to SZ twisting processes and SZ twisting machines by means of which two to five elements to be twisted, for instance, conductors for light PVC-sheathed cables, conductors for pairs, triplets or spiral quads of communication cables, or spiral quads for base bundles of communication cables can be twisted together without problem. A characteristic of all developed SZ twisting methods working with a concentrated or unconcentrated longitudinal accumulator is that the final twist of the stranded material results from a superposition of at least two oppositely directed twisting processes which take place at the input and output of the SZ twisting machine. This applies, for instance, to twisting with a rotating longitudinal accumulator of alternating direction of rotation (U.S. Pat. No. 3,169,360), to twisting with a revolving twisting device and changing accumulator content (U.S. Pat. Nos. 3,481,127; 3,797,217), to twisting by means of a twisting head (twisting closer) arranged at the end of a torsion section (U.S. Pat. No. 3,593,509) as well as to twisting with two twisting heads which are arranged at the beginning and end of a torsion section and revolve with constant direction of rotation (U.S. Pat. No. 3,823,536).
In the SZ twisting of five or more elements, difficulties can arise due to the fact that the elements to be twisted are arranged in one or more layers about a core in the finished stranded unit since then the absolute length of the elements to be twisted in the layer or layers is greater than the length of the twisted elements arranged in the core. Under the influence of the twisting operations executed one after the other during the SZ twisting, this leads to relative length differences between the core and the outer layer to the twisted material, which can result in serious disturbances in the twisting process. If, for instance, a shorter length of lay is produced in the last twisting operation than in the first twisting operation, then the core of the strand is upset and has a tendency to emerge to the outside between the elements of the outer layer.